The present invention relates to an improvement for a certain class of digitizers. The improvement allows increased digitizing accuracy while the cursor or stylus is in motion over the platen or digitizing surface. In general, the invention pertains to the class of digitizers wherein the X (abscissa) and Y (ordinate) coordinates are measured in alternation and at separate times. It also applies to a particular type of coordinate measurement technique wherein the individual X or Y measurements are themselves each compounded of two component measurements performed separately and in sequence.
The evil that arises in the general case of alternating separate X and Y measurements is that the motion of the stylus or cursor can create a situation where the reported (X, Y) coordinates do not lie on the actual path taken by the stylus. One solution to this problem is discussed in U.S. Pat. No. 4,255,617 issued to Carau, et. al. FIGS. 19 through 21 of that patent address the problem and one solution thereto. The solution described in Carau's FIGS. 20 and 21 is to find the cursor velocity in one coordinate direction, and correct the value of that coordinate by the amount of cursor travel during the time required to measure the other coordinate. It would be desirable in the interests of speed and simplicity to achieve a comparable correction to the measured coordinates without resorting to actually finding the velocity. The instant invention accomplishes this objective by altering the measurement sequence to be X, Y, X (or perhaps Y, X, Y) and simply averaging the first and third measurements. This produces a value that is correctly associated with the middle intervening measurement, provided that the existence of certain conditions can be assured. These include uniform cursor motion (which will be assumed) and equal measurement times for each of the measured X and Y values.
The invention is also applicable at a different level to a particular type of coordinate measurement technique. In a digitizer using such a technique, each coordinate (say, X) is compounded from two similar components (say, X' and X"). X' and X" are not "coarse" and "fine," as in some digitizing techniques. Instead, they are both high resolution fine measurements, a comparison of which determines the number of modulo fine values to be added to the fine component (which could be either of X' or X"). This measurement technique will be set out in more detail in a later portion of the Specification. In summary, however, it may be pointed out that stylus motion disturbs the validity of the X' and X" measurement pair in much the same way that it introduces a dissimilarity between X and Y: X' is no longer correctly associated with X" because they were measured at different times. This difficulty is overcome by modifying the component measurement sequence for X to be X', X", X', and then associating X" with the average of the two X' values. The same is also done for Y: the measured Y components are Y', Y", Y', and Y" is associated with the average of the two Y' values.